The purpose of this study is to continue quantitative and qualitative investigations into the nature of energy metabolism in the fetus. A subhuman primate model (the fetal baboon) will be used to extend previous studies of quantitative energy metabolism. Utilizing appropriately placed arterial and venous catheters for sampling of arteriovenous metabolite differences, antipyrine infusion and the radioactive microsphere technique for measuring fetal blood flow and specific organ blood flows, substrate uptake and production by the primate fetus and by specific tissues of the fetus will be evaluated. These studies will be enhanced by similar investigations of specific organ substrate uptake and production in a more chronic fetal lamb preparation.